This invention relates to active matrix pixel devices and their method of fabrication, and more particularly, but not exclusively, to the customisation of active matrix displays, and especially active matrix liquid crystal displays (AMLCDs), where a range of pixel pitches can be achieved from an initial universal active matrix.
Examples of active matrix pixel devices other than display devices include sensing devices such as image sensing devices and fingerprint sensing devices in which the matrix elements comprise for example optical or capacitance sensing elements, transducer devices, in which the matrix elements comprise moveable electromechanical elements, for example piezoelectric or electrostatically controlled actuator elements.
Active matrix pixel devices, such as AMLCDs, are used in an increasingly wide variety of products, including consumer electronics, computers and communication devices. Such devices are often included in portable products where the size and compactness of the product are particularly important considerations.
An example of such a device is described in EP-A-0617310. In this device, a row and column matrix array of display pixels is provided, each of which is driven via an associated switching element in the form of a TFT (thin film transistor). As is usual, the device comprises a layer of liquid crystal (LC) material disposed between a pair of spaced substrates carrying electrodes which define individual display pixels. The TFTs are carried on the surface of a first substrate together with sets of row, (scanning), conductors and column, (data), conductors through which the TFTs are addressed for driving the display pixels. Each TFT is disposed adjacent the intersection between respective ones of the row and column conductors. The gates of all the TFTs associated with a row of display pixels are connected to a respective row conductor and the sources of all the TFTs associated with a column of pixels are connected to a respective column conductor. This forms an array of active cells in which each cell comprises a TFT having associated row and column conductors. An array of reflective metal pixel electrodes is carried on an insulating film which extends over the first substrate and covers the TFTs and the sets of address conductors so that the pixel electrodes are positioned generally above the level of the TFTs and the address conductors. As is conventional, each pixel electrode is associated with one respective TFT. Each individual pixel electrode is connected to an underlying contact electrode, which is integral with the drain electrode of its associated TFT, through a respective opening formed in the insulating film directly over the contact electrode. With this type of construction, in which the array of pixel electrodes and the array of TFTs are provided at respective different levels above the substrate surface, the pixel electrodes can be enlarged such that at two opposing sides they extend slightly over adjacent row conductors and at their two other opposing sides they extend slightly over adjacent column conductors, rather than being sized smaller than the spacing between adjacent row conductors and adjacent column conductors with gaps provided between each edge of the pixel electrode and the adjacent conductor, as in display device arrangements in which the individual pixel electrodes are arranged substantially co-planar with, and laterally of, the TFTs. In this way, therefore, the pixel aperture is increased and in operation more light which passes through the LC layer and reaches the pixel electrode is reflected back to produce a brighter display output. Moreover, parts of a deposited metal layer which is patterned to form the reflective pixel electrodes can be left immediately overlying the TFTs during the patterning process so as to act as light shields for the TFTs to reduce photoelectric effects in the TFTs due to light incident thereon, thereby avoiding the need to provide black matrix material on the other substrate for this purpose. The substrate carrying the TFTs, address conductors and pixel electrodes constitutes the active plate of the display device. The other, transparent, substrate constitutes the passive plate and carries a continuous transparent electrode common to all pixels in the array and an array of colour filter elements corresponding to the array of pixels with each filter element overlying a respective pixel electrode.
In order to supplement the capacitance of each LC cell in an AMLCD, a storage capacitance is commonly provided in parallel with the LC cell. This is required to maintain the desired voltage across the cell when the driving signal is removed. One example method of providing a storage capacitor is to form an extra conductive layer over the contact electrode with a dielectric material sandwiched in between.
Conventionally, the active matrix array is fabricated by depositing on the substrate various layers of conductive, insulating and semiconductive layers and patterning these layers using a photolithographic definition process involving photolithographic masks that determine the pattern of individual layers.
There is growing interest in making AMLCDs which can be easily, quickly and cheaply customised to the needs of a particular application. For example, different customers may require displays having different and specific pixel pitches. This is especially true for small/mid-sized displays for applications in portable products, such as mobile phones, PDAs, and the like, where the market is characterised by constant change in product ranges and hence display designs.
It is well known in the art that to produce a batch of displays to a new design requires a complete new set of photolithography masks. A problem is that this can make customised AMLCDs rather expensive, as the investment costs in a mask set are high and the total number of displays over which these costs can be recovered may be quite small. In addition, lead times can be long as masks must be designed for each specific customer requirement and each customer specific product must then be processed through to completion.
It is an object of the present invention to provide an improved method of producing an active matrix pixel device.
It is another object of the present invention to provide a method of producing an active matrix pixel device allowing for a cost reduction in meeting customer specific requirements.
According to one aspect of the present invention there is provided a method of constructing an active matrix pixel device comprising:
providing a universal active matrix comprising on a substrate a matrix array of switching elements whose spacing defines a base pitch and sets of row address conductors and column address conductors for addressing the switching elements;
forming on the substrate a dielectric layer over the array of switching elements,
forming an array of contact holes in the dielectric layer such that contact can be made with a plurality of switching elements,
forming a pixel array on the universal active matrix, the pixel array comprising a matrix array of pixel electrodes in electrical contact with underlying switching elements via the contact holes, the spacing of the pixel electrodes defining a pixel pitch,
wherein the pixel pitch is greater than the base pitch.
When the universal active matrix (UAM) has been formed on the first substrate, the partly constructed device can be stockpiled if required. The pixel array with a desired pixel pitch can then be formed at a later stage to meet the customers"" requirements. One advantage that the invention provides is that a large proportion of the production process can be carried out before the customisation. Such a method enables active matrix pixel devices of differing designs, e.g. having different pixel electrode layouts, to be fabricated more readily, and less expensively, than previously. Using a common UAM, which can be stockpiled for convenience, enables a reduction in the time between the customer ordering the device and the completion time. Another advantage is that less mask sets are required for each new custom active matrix pixel device as the same masks can be used for each UAM. This reduces the cost of customised devices.
According to another aspect of the invention there is provided an active matrix pixel device comprising a universal active matrix having a matrix array of switching elements arranged so as to define a base pitch, and a pixel layer having a matrix array of pixel electrodes arranged so as to define a pixel pitch, wherein the pixel pitch is greater than the base pitch.
The switching elements of the UAM are preferably addressed by respective row and column address conductors as described previously. Each switching element with its respective row and column address conductors forms an active cell. The spacing of the active cells defines the base pitch. The spacing may not necessarily be the same in the horizontal (row) and vertical (column) directions and so the base pitch can be sub-divided into horizontal and vertical components if necessary. The base pitch is preferably made as small as is conveniently possible. This defines the minimum achievable pitch of the final active matrix pixel device. By minimising the base pitch in this way the range of achievable pixel pitches for the final product is increased.
In a preferred embodiment a dielectric layer overlies the array of switching elements, preferably comprising a polymer material and covering the whole array. This acts to reduce significantly capacitive coupling between the overlying pixel electrode layer and the underlying UAM.
The switching elements preferably comprise thin film transistors (TFTs) as known in the art. The TFTs may be top-gate or bottom-gate type TFTs. TFTs in matrix arrays are normally fabricated as respective, separate, semiconductor islands of amorphous, polycrystalline or microcrystalline silicon material or a plastics organic material defined by patterning a continuous semiconductor layer deposited over the substrate to leave discrete areas of semiconductor material arranged in a row and column matrix.
As the pixel pitch is greater than the base pitch, some pixel electrodes may cover more than one switching element. Electrical contact can be made with at least one of the covered switching elements via the contact holes in the dielectric layer. In a preferred embodiment of the present invention each pixel electrode is connected to and controlled by just one of the underlying switching elements. Therefore some switching elements are left redundant and are not used to supply signals to the pixel electrodes. An advantage of using only one switching element per pixel electrode is that there is less capacitive coupling between the row and column address conductors. This helps to reduce display artefacts such as cross-talk and flicker.
In another embodiment individual pixel electrodes may be addressed simultaneously by more than one underlying switching element. This can be done by connecting the associated adjacent row and column conductors together in parallel.
Although the base pitch and pixel pitch are substantially unrelated it is envisaged that there may be an integral relationship in which the pixel pitch is an integer multiple of the base pitch in both the horizontal and vertical components thereof. For example, where the horizontal pixel pitch is three times the horizontal base pitch and the vertical pixel pitch twice the vertical base pitch, each pixel electrode covers six switching elements. Therefore any one or more of these switching elements can be utilised if necessary to operate their associated overlying pixel electrode. According to a further aspect of the present invention the active matrix pixel device comprises an AMLCD device. In preferred embodiments the AMLCD device comprises a reflective or transflective type of display device.
Although the active matrix pixel device according to the present invention preferably comprises a liquid crystal display it is envisaged that the invention can be applied to other types of active matrix display devices, for example electrophoretic, electrochromic or electroluminescent display devices, and also to active matrix pixel devices for non-display purposes, for example sensor arrays such as image sensing arrays, etc.